An IC driver, namely, a voltage output amplifier such as for a liquid crystal display device and an organic EL display device is required to have a large capacity and to have low power consumption and a high speed output response characteristics. For this reason, the output voltage is required to have a dynamic range from a ground potential to a power source potential.
The present applicant includes a buffer circuit having an output acceleration circuit (an enhancer circuit) in which at an output side of a CMOS buffer amplifier and between the output side and a power source line as well as between the output side and a ground line respective switch circuits are provided and further, two comparators which compare an input voltage and an output voltage of the buffer amplifier are provided (patent document 1). The buffer circuit is for enhancing a through rate of the output voltage signals by feeding from the output acceleration circuit much amount of the output current at the time of the leading up and trailing down of the buffer amplifier.
In the buffer circuit, an offset is provided for the comparison operation of the respective comparators, when the input voltage to the buffer amplifier is in a predetermined range exceeding a predetermined offset voltage with respect to the output voltage, the same is detected by the comparator, when an input signal is led up, the output voltage signal is rapidly led up by turning ON the switch circuit connected to the power source side, when an input signal is trailed down, the output signal is rapidly trailed down by turning ON the switch circuit connected to the ground side, and when the output voltage falls in the range of the offset voltage with respect to the input voltage, only the output of the buffer amplifier is used so that the output voltage corresponding to the input voltage is output from the buffer amplifier.
Further, the offset voltage provided for the comparison operation of the comparators is formed by making use of a threshold value of a pair of MOS transistors, which constitute the comparators.
Patent Document 1: JP 2004-140487 A
The two switch circuits connected respectively to the power source side and the ground side as disclosed in JP 2004-140487 A are ON/OFF controlled by the respective comparators. In this instance, one of the comparators serves as a current switch circuit which corresponds to the leading up side and is constituted by a differential pair of N channel MOS transistors and the other of the comparators serves as another current switch circuit which corresponds to the trailing down side and is constituted by a differential pair of P channel MOS transistors. For this reason, the respective comparators have a dead band (a voltage between source and gate) of about 0.8 V, which is determined by an operational threshold value of the N channel or P channel transistors.